I have a messenger type bag from
Celltei.com and I love it. Tuffy weighs about 7 lbs and he fits comfortably into a size small and I can also carry him and Pixie together in there which is about 10 lbs combined. Here's a picture of mine:
http://www.pluschic.com/wmTuffy%20013.jpg
I've also come across these sling bags:
Urban Messenger Carrier Poochie Pouch Carrier (I've also seen this one on eBay for about $20 cheaper too)